This invention relates to sonic borehole logging tools and methods and more particularly, to sonic borehole logging using a continuous wave input signal.
Sonic borehole logging is an accepted method of measuring the mechanical properties of material around a borehole by measurement of velocities of various elastic waves which travel through the material and surface waves guided by the borehole material interface. Such logging basically involves the injection of acoustic energy into a borehole wall at one point and the reception of part of that energy which has been transmitted back into the borehole at another point. With a known distance between transmitter and receiver, the time lag between transmission and reception can be used to indicate acoustic energy velocity and other parameters. More accurate equipment has a single transmitting transducer and two receiving transducers with the time measurement being made between the reception of signals at the two receiving transducers which are spaced apart a known distance.
Most known borehole logging tools use an impulse-type transmitted signal which is detected as a series of wavelets at the receiving transducers. Such conventional pulsed techniques require subjective operator intervention in such matters as choosing correlation windows, thresholds, etc. and this prevents such techniques from being truly reliable automated velocity measurement methods, especially for waves other than the compressional first arrival. It would be desirable to have a logging device which can indicate such travel times reliably without subjective operator intervention.